1. Field of the Invention
The invention concerns a metallic strip gasket and, in particular, a metallic strip gasket for use in connecting detachable metallic shielding elements in a high-frequency-tight manner.
2. Description of the Prior Art
When employing functional electrical modules which are arranged in housings, e.g., so-called electronics cabinets, it is often necessary to shield these functional modules against electromagnetic waves present in the external space or to confine the electromagnetic waves emanating from these functional modules to the interior of the housing, in order to prevent interference effects on sensitive electronic modules arranged in the vicinity of the housing. Thus, in the case of a process computer installed in a power station, the computer housing must act as a shield against the electromagnetic high-frequency waves present in the external space, thereby attenuating such waves to a level which is harmless for the functioning of the computer. Similarly in the case of high-frequency generators arranged in a housing, it is necessary to design the housing so that it acts as a shield to limit to a minimum the electromagnetic radiation emanating from the housing. In order to realize such shielding action, the aforesaid housings are typically formed of a metallic material so that the housings operate in the manner of a Faraday cage. Moreover, to achieve maximum shielding, the frame and the walls of the metallic housing must be designed so as to make the best possible contact. While the shielding effect attainable by a normal metallic housing is in the order of 20 dB, this effect can be increased to 90 dB by welding the joints of the housing shut on all sides. Such a welded shut housing, however, would be of limited usefulness, since it would be expensive to manufacture and would not permit ready accessibility, which is absolutely necessary in most applications.
In order to provide a metallic housing with the desired attenuation properties without having to weld the housing joints, the parts of the housing which are in contact with each other can be provided with surfaces which ensure good metallic contact. The cost of providing such surfaces, however, is about 60% of the total cost of the housing. As only a small percentage of the housings used must meet stringent shielding requirements, there appears to be little reason to standardize housing production so as to provide all the components of such housings with metallically highly conducting surfaces at the joints. On the other hand, to individually fabricate such high-attenuation housings would be extremely costly.
One inexpensive technique for connecting adjoining shielding elements of a housing in a high-frequency-tight manner is to place specially designed metallic sealing elements in the gap between the shielding elements, i.e., for instance, between the shielding plates and the frame of the housing. These sealing elements provide a contact which is as low-resistant and continuous as possible between the adjoining shielding elements when pressed against the walls of elements forming the gap.
A particular type of sealing gasket used for the above purpose is described in Swiss Pat. No. 520,460. The latter sealing gasket comprises a contact sheet which is provided with cross cuts arranged in rows. The four triangular tabs produced by a cross cut are pushed out from the plane of the sheet so as to be inclined at an angle of preferably 45.degree. with the plane. The tabs of adjacent cross cuts, moreover, are inclined toward different sides of the gasket. Thus, as can be appreciated, the rows of cross cuts result in a multiplicity of tabs, the tabs always being arranged in groups of four each generated by a particular cross cut. Within each such group of four tabs, there are always two pairs of tabs whose projections on the plane of the gasket are rotated relative to each other by 180.degree..
The cut edges and, in particular, the tips of the triangular tabs of the aforesaid sealing gasket serve to dig resiliently into the adjacent surfaces of the shielding elements between which the gasket is placed. By making the cross cuts longer, an improvement in the resilient spring action of the tabs is achieved. This is particularly important if the gasket is to remain re-usable for a repeated assembly after the shielding elements are mounted. Moreover, the resilient contact of the tabs yields an attenuation characteristic which is constant over extended periods of time, which is of significant importance in the design of shielding elements.
When the aforesaid gasket is clamped between the adjacent walls of the shielding elements, the spring tabs projecting from the plane of the gasket are pushed back, at least partly, into the gasket plane. This tab movement will be referred to hereinafter as "springback." In the region of a very small area in the immediate vicinity of the intersection point of each cross cut, a contact is then formed between the four tips of the tab group and the adjacent shielding element. The next such contact between the sealing gasket and the shielding element then takes place in the vicinity of the intersection point of the next cross cut. However, since the cross cuts must exceed a certain minimum length to ensure sufficient spring action of their resultant tabs, the individual cross cuts have to be spaced relatively far apart from each other. Thus, only a relatively low contact density between the sealing gasket and the shielding elements is ever realized. As a result, considerable path resistances confront the current flowing between the shielding elements via the gasket. This, in turn, has an adverse effect on the optimum shielding effectiveness provided by the gasket in the range of low frequencies. A partial correction of such poor low frequency performance can be made by coating the surface of the gasket with high-conductivity material. While the low contact density between the gasket and the shielding elements, thus, promotes a poor shielding effectiveness at low frequencies, it likewise produces a similar result at frequencies considerably higher than 1 MHz. The aforesaid prior art gasket can so preferably be applicated in the frequency range below 1 MHz. Modern applications, however, do require effective shielding over a larger bandwidth than that provided by the aforesaid prior art gasket.
It is therefore a primary object of the present invention to design a high-frequency gasket of the above type having optimized attenuation properties and, therefore, a maximum shielding effectiveness.